Enhanced geothermal systems (EGS) are engineered deep rock heat exchangers that require hydraulic fracturing to create artificial heat storage and transfer heat through fissures. The technique involves a complex heat-fluidsolid (THM) coupling process. In this study, based on the Frontier Observatory for Research in Geothermal Energy (FORGE) site 16A(78)-32 horizontal well in Milford, Utah, USA, a hydraulic fracturing model for horizontal wells, distinct from traditional EGS doublet designs, was established. This model combines the three thermal-fluid-mechanical (THM) physical fields and investigates the impact of injection volume, temperature, and injection sequence on ground rock modification during horizontal well hydraulic fracturing. The results show that after 75 h of enhanced production treatment, the model predicts an increased production volume of approximately 8.3 x 106 m3. Increasing the injection fluid volume stimulates shear fracturing in existing fractures, widening the area of greater production. Changing the injection sequence does not increase the volume of fractured rock. With increasing injection temperature, there is a slight decrease in the modified volume produced.

As a clean and renewable energy, geothermal is gaining widespread attention worldwide. The development of hydrothermal resources is accompanied by the coupling processes of thermo-hydro-mechanical-chemical (THMC) caused by production and injection, which have a complex effect on the porosity and permeability of reservoir. Exploring the effects of these processes on the changes in various physical fields within the reservoir is of great significance for the efficient and sustainable development of geothermal energy. Based on the fully coupled framework of fluid flow and transfer in TOUGH2, a method that considered both mechanical and chemical processes was proposed and taken in this study, to analyze the effects of long-term injection and production on reservoir porosity and permeability. The results show that the effective stress changes under fluid pressure and thermal stress, as well as mineral dissolution and precipitation under chemical action, jointly led to the variation of reservoir porosity and permeability. The overall variation trend was mainly controlled by the mechanical action, as the influence of fluid pressure was more obvious and rapid, which dominated the mechanical changes of the whole reservoir in the early stage of injection (within 3.5 years). With the continuous injection, thermal stress played a more obvious role around the cooling area. By 30 years, the significant change in effective stress has extended to the entire reservoir. In the later stage of operation (after 20 years), as the temperature field of the reservoir gradually stopped changing, porosity began to increase under the dominance of fluid pressure. The effect of chemical action was long-term and lag in time. Mineral precipitation only occurred near the injection well, and the precipitation range and amount increased gradually with the continuous injection. By 30 years, the precipitation of dolomite injected into the grid reached 0.06%.

Geothermal energy is a type of renewable energy that has rich reserves, is clean, environmentally friendly and has been widely used in the heating industry. The single-well closed-loop geothermal system is a technology with the characteristics of “taking heat without taking water” and is mainly used for geothermal energy heating. Although the heating requirements in the cold region of Northeast China are urgent, the traditional heating mode not only has high economic costs but also causes serious damage to the environment. Therefore, it is of important practical significance to change the heating structure and develop and utilize geothermal energy for heating according to local conditions. In this study, the actual operating single-well geothermal system in the Songyuan area of Jilin Province is used as a case study, and a numerical model is established based on the T2WELL simulation program. The flow production temperature and heat extraction response law of the single-well system in the M1 and M2 wells are contrasted and analyzed under the three key factors of geothermal gradient and injection temperature and flow rate. Based on the simulation results, an optimized development and utilization plan for the M1 and M2 wells is proposed. These results provide a theoretical reference and heating potential evaluation for the promotion of single-well geothermal systems in Northeast China. Taking the geothermal gradient of 4.2 ° C/hm as an example, after 30 years of operation, the heat extraction of the M1 well is 406 kW, and that of the M2 well is 589 kW. Compared with the M1 well, although the M2 well has higher heat extraction, the radial variation in reservoir temperature is more than 50 m under long-term operation, which is not conducive to long-term development and utilization.

为进一步探究山东埕宁隆起区域内热储资源量,制定合理可持续的开发方案,为地热资源的进一步勘探和综合开发提供科学依据,本次研究结合野外地质调查测试结果及以往成果资料,分析了区内地热流体成因,并采用热储法与综合指数法,对寒武系—奥陶系岩溶热储进行资源量估算及开发前景分析.结果表明:区内寒武系—奥陶系岩溶热储可利用地热资源量为1.97×1019 J,折合标准煤6.72×108 t;地热流体年均可采量为29 777.40万m3/a;具备未来开采条件的区域面积为1 538.29 km2,占岩溶热储总面积的86.18％.区内寒武系—奥陶系岩溶热储具有较高的开发利用潜力,应加大勘查力度,推进开发利用进程.

An enhanced geothermal system is a kind of artificial geothermal system, which can economically exploit geothermal energy from deep thermal rock mass with low permeability by artificially created geothermal reservoirs. Chemical stimulation refers to a reservoir permeability enhancement method that injects a chemical stimulant into the fractured geothermal reservoir to improve the formation permeability by dissolving minerals. In this study, a reactive solute transport model was established based on TOUGHREACT to find out the effect of chemical stimulation on the reconstruction of a granite-hosted enhanced geothermal system reservoir. The results show that chemical stimulation with mud acid as a stimulant can effectively improve the permeability of fractures near the injection well, the effective penetration distance can reach more than 20 m after 5 days. The improvement of porosity and permeability was mainly caused by the dissolution of feldspar and chlorite. The permeability enhancement increased with the injection flow rate and HF concentration in the stimulant, which was weakly affected by the change in injection temperature. The method of chemical enhancement processes can provide a reference for subsequent enhanced geothermal system engineering designs.

The key to ensuring the economic feasibility of EGS mainly includes two points. On the one hand, it is necessary to ensure the connectivity of the artificial fracture network; on the other hand, it is necessary to determine the most efficient geothermal energy exploitation mode. Most previous studies have only focused on one of the points. To restitute the entire geothermal energy development process, the two parts should be combined to conduct research. In this study, a random fractured medium model was established based on the TOUGH2-BIOT simulation program and the whole process of reservoir stimulation was analyzed. According to the results of reservoir stimulation, different geothermal energy exploitation schemes are set up, and the heat transfer efficiency of the conventional double vertical wells, the horizontal wells, and the double-pipe heat exchange system are comparatively analyzed. The results show that reservoir reconstruction is mainly divided into three stages: In the first stage, the hydraulic aperture of the conducting fractures reaches the maximum value; in the second stage, the non-conductive fractures overcome the in situ stress and become conducting fractures; in the third stage, the rock in the reservoir undergoes shear failure, the fractures expand and connect, and finally, a fracture network is formed. After each stage, the volume of the enhanced permeability area is approximately 10,000, 21,000, and 33,000 m3, respectively. After 30 years of exploitation, the outlet temperature and thermal power output of conventional double vertical wells are the highest, while the horizontal wells have the highest heat extraction ratio. The temperature of a production well in the conventional double vertical wells model, horizontal wells, and double-pipe heat exchange system is 101 °C, 93.4 °C, and 91.6 °C, a decrease of 41.2%, 45.7%, and 46.7%, respectively. The thermal power output is 6.67 MW, 6.31 MW, and 6.1 MW, a decrease of 39.4%, 42.6%, and 44.5%, respectively. The heat extraction ratio of the horizontal wells is 2% higher than the double-pipe heat exchange system and 6.5% higher than the conventional double vertical wells.

The injection of circulating fluids (usually water) is the predominant means of heat extraction in an enhanced geothermal system (EGS); however, the additional injection of circulating fluids can cause a water-rock reaction with a dry thermal rock mass, resulting in the generation of a large number of secondary minerals. These secondary minerals cause fracture closure in artificial reservoirs and severely impede the sustainable use of these geothermal systems. Therefore, this study combined laboratory tests with a PHREEQC hydrogeochemical simulation. First, the changes and mechanisms of mineral and aqueous chemical fractions in the artificial reservoir fractures were analyzed after using groundwater from the sandstone aquifer at the Frontier Observatory for Geothermal Energy Research (FORGE) site as a heat transfer fluid in the FORGE project. Second, the dissolution patterns and precipitation of minerals commonly found in artificial fractures were simulated in four different environments with the aid of PHREEQC after completing transconversion. Finally, the effects of different fluids were evaluated for the unblocking of typical minerals. The results show that as a heat transfer fluid, groundwater causes the dissolution of K-feldspar, albite, and illite, partial decomposition of kaolinite and proto-silicate, rapid production of anorthite, decomposition of dolomite, sodization of Ca-montmorillonite, and dolomitization of black mica. Mud acid was more effective in unblocking fissure closures caused by anorthite, chlorite, and kaolinite. NaOH was more effective in unblocking fissure closures caused by illite and Camontmorillonite. Groundwater is not suitable as a blockage remover and can easily produce secondary mineral blockages (calcite and aragonite). The effect of ultrapure water is relatively stable; it can be used as a heat exchange fluid or blockage remover, but when used as a remover, it behaves primarily as a physical-mechanical action process.

During the operation of an enhanced geothermal system (EGS), the non-equilibrium temperature, pressure, and hydrochemistry caused by fluid injection intensify water–rock interactions, induce the mineral dissolution and precipitation in the reservoir near an injection well (also referred to as the near-well reservoir), and change reservoir permeability, thus affecting continuous and efficient geothermal exploitation. Based on the investigation of the M-1 injection well of the EGS in the Matouying uplift of Hebei Province, China, a THC reactive solute transport model using the TOUGHREACT program was established in this study to explore the mineral dissolution and precipitation laws of the near-well reservoir and their influencing mechanisms on the reservoir porosity and permeability in the long-term fluid injection of this well. As indicated by the results, the dissolution of primary feldspar and chlorite and the precipitation of secondary minerals (mainly dolomite and illite) occurred and water–rock interaction significantly reduced the porosity and permeability of the near-well reservoir in the long-term continuous injection process. Appropriate reduction in the injection flow rate, injection temperature, and the Mg2+ and K+ contents in the injected water can help inhibit the formation of secondary minerals and delay the plugging process of the near-well reservoir.

The abundant geothermal energy in hot dry rock (HDR) geothermal reservoirs is an attractive renewable energy resource with great potential. Cyclic injection in hydraulic fracturing has been proved to be a suitable way for the geothermal energy exploitation. However, the fracture initiation and propagation regimes induced by cyclic injection have been inadequately studied, and the fracturing optimization for HDR is often by experience. For this reason, true tri-axial hydraulic-fracturing tests were conducted to study the initiation and propagation of hydraulic fractures, while the specimens were subjected to the cyclic injection with different cycle time duration and injection rate. The results revealed that the initiation of hydraulic fracture was displayed as three basic patterns, and these fracture-initiation patterns were developed into two fracture geometries: simple fracture with only 2-4 strands and complex fracture network with >4 strands. The main reason for this difference is that the initiation and propagation of hydraulic fracture is controlled by the way of fluid circulation, and the cycle time duration and injection rate each played a different role in hydraulic fracturing. At a low injection rate, the initiation and propagation of hydraulic fractures under the high-frequency cycle (cycle time duration = 10 and 20 s) are mainly controlled by the change of injection pressure. Here, the hydraulic energy mainly acts on the rock near the well. The frequent change of injection pressure promotes the initiation of micro-fractures and forms a complex but short hydraulic fracture network. However, when high injection rate is subjected, too high and too low cycle frequencies (cycle time duration = 5 and 40 s) both tend to form a simple fracture, but the fracture has strong extension ability. With that, the similarity criterion of physical phenomena between the on-site prototype and the experimental model were discussed, and these lab-scale results were translated into the fracturing site of HDR.

AbstractThe development of sandstone-type geothermal energy is an important part of the development of geothermal resources and has great significance in promoting environmental protection and energy structural transformation. In sandstone geothermal energy development, recharging is the main method to ensure bottom hole pressure. However, the pressure and temperature changes of sandstone reservoirs under recharge conditions have not been extensively studied. It is easy to ignore the hydraulic relationship between the production and the injection wells, which leads to an increased risk of thermal breakthrough. Therefore, a three-dimensional hydrothermal coupling model is established, and simulation studies of different flow rates, well lengths, and well spacings are completed in this paper. Here, we show the numerical simulation results. The low temperature expansion zone and hydrostatic pressure near the injection well increase with increasing flow rate, and the maximum expansion of the low temperature zone is about 350 m. The low temperature expansion area near the injection well has a small relationship with the well spacing, and the increase in hydrostatic pressure is proportional to the well spacing. As the length of the well increases, the increase in hydrostatic pressure near the injection well decreases, indicating that the injected water under the long well section easily enters the reservoir. When no thermal breakthrough occurs and the hydrostatic pressure drops significantly near the production well, it is recommended that the flow rate be controlled at approximately 20–25 L/s, the well spacing should be 600–800 m, and the well length should be greater than 100 m.

Chemical stimulation has been increasingly applied to improve the performance of geothermal reservoirs since early 1980s. The potential for the successful application of this technique to high-temperature reservoirs hosted in granitic rocks is still uncertain, and one of the keys to assess this potential is to investigate experimentally the geochemical reactivity induced via chemical stimulation on relevant rock specimens. On this premise, we combined high-temperature and high-pressure dynamic simulation and static corrosion experiments to explore the effect of different chemical stimulants on the permeability of granite samples from the Baimiao formation (Hebei Province, China). Experimental results show that NaOH-dominated alkaline stimulants cause only weak dissolution patterns on primary feldspar and quartz, and they do not sensitively affect the original amount of chlorite. The overall effect is a negligible enhancement of the original permeability of all the granite specimens analyzed. Conversely, a large increase in permeability is observed when an acid mixture of 10wt% HCl + 1.5wt% HF is used as a stimulant, with an observed maximum magnifying effect of about 27 times, due to the effective dissolution of feldspar and chlorite. Though quartz appears not to be affected by dissolution, a relatively large amount of secondary neo-formed amorphous silica is also documented.

为了延长增强型地热系统的使用寿命,实现地热能高效可持续开发,需要研究储层堵塞物的类型、来源及堵塞机理,并分析现有解堵液体系的解堵原理和特点.为此,系统总结了国内外典型增强型地热系统储层的堵塞机理和解堵技术研究的最新进展,对比分析了不同解堵技术的适用条件及优缺点,梳理了增强型地热系统应用解堵技术的工程经验.研究结果表明:①矿物颗粒脱落运移堵塞(物理堵塞)和次生矿物堵塞(化学堵塞)是人工裂隙堵塞物的主要类型,这些堵塞物可以通过物理、化学或物理—化学混合解堵技术解除;②化学解堵技术比物理解堵技术解堵效果更加理想,新型酸液体系的缓释性和转向性研究将是未来酸性解堵液开发的主要方向;③混合解堵技术结合了物理和化学解堵技术的优点,在全球应用最为广泛,新型脉冲技术和高效解堵液的研发将对混合解堵技术的发展产生重要影响.结论认为,基于高效酸性解堵液研发的混合解堵模式将成为解决增强型地热系统储层堵塞最具发展潜力的解堵关键技术,该技术可为未来中国干热岩储层维护和深部地热能持续开发提供有力支撑.

The Enhanced Geothermal System (EGS) is an artificial geothermal system that aims to economically extract heat from hot dry rock (HDR) through the creation of an artificial geothermal reservoir. Chemical stimulation is thought to be an effective method to create fracture networks and open existing fractures in hot dry rocks by injecting chemical agents into the reservoir to dissolve the minerals. Granite is a common type of hot dry rock. In this paper, a series of chemical stimulation experiments were implemented using acid and alkaline agents under high temperature and pressure conditions that mimic the environment of formation. Granite rock samples used in the experiments are collected from the potential EGS reservoir in the Matouying area, Hebei, China. Laboratory experimental results show that the corrosion ratio per unit area of rock is 3.2% in static acid chemical experiments and 0.51% in static alkaline chemical experiments. The permeability of the core is increased by 1.62 times in dynamic acid chemical experiments and 2.45 times in dynamic alkaline chemical experiments. A scanning electron microscope analysis of the core illustrates that secondary minerals, such as chlorite, spherical silica, and montmorillonite, were formed, due to acid-rock interaction with plagioclase being precipitated by alkaline-rock interactions. Masking agents in alkaline chemical agents can slightly reduce the degree of plagioclase formation. A chemical simulation model was built using TOUGHREACT, the mineral dissolution and associated ion concentration variation being reproduced by this reactive transport model.